A comprehensive study of violence in movies rated PG-13 has found that gunplay has tripled in such films since 1985, when the rating was introduced, and further concluded that from 2009 to 2012, PG-13 films have contained as much or more violence than films rated R.

The study, published Monday in Pediatrics, the official journal of the American Academy of Pediatrics, offers a troubling portrait not only of the accelerating levels of violence in blockbuster movies aimed at children but also the effect this kind of cinematic bloodshed can have on young moviegoers, which other studies have shown can increase hostile behavior.

“This research suggests that the presence of weapons in films might amplify the effects of violent films on aggression,” says the report, authored by academics at Ohio State University, VU University Amsterdam and the Annenberg Public Policy Center at the University of Pennsylvania.

The study examined 945 top-grossing films from 1950 to 2012, in which coders identified 17,695 violent sequences. The sequences in which guns were used were tallied separately. Researchers found that a staggering 94% of the highest-grossing films since 1985 had one or more five-minute sequence containing violence. Of those 396 films, there were 783 segments of gun violence, with violence growing the fastest within the PG-13 rating. In 2012, the study found that the level of gun violence in PG-13 films surpassed that of R-rated releases.

“Films with a PG-13 rating are popular, accounting for more than one-half of top-grossing film revenue, but unfortunately they are not restricted at movie theaters to youth,” the study said. “Film producers may therefore be strengthening the weapons effect by increasingly including guns in scripts that involve violence in the films youth are most likely to see.”

The Motion Picture Assn. of America, which administers the movie ratings, declined to comment on the report.

Citing earlier studies that have examined links between violent entertainment and reciprocal behavior from its consumers, the researchers said such causal relationships could be magnified by easy online access to PG-13 productions.

“The presence of guns in films also provides youth with scripts on how to use guns. In addition, children no longer need to go to movie theaters to see films; films are readily available on the Internet or cable,” the report said. “Thus, children much younger than 13 years can easily view films that contain ample gun violence.”